A sealed electrochemical storage cell or secondary cell (both terms being equivalent, we shall use the term secondary cell in this specification) conventionally comprises an electrode plate group made up of alternating positive and negative electrodes on either side of electrolyte-impregnated separators. Each electrode is made up of a metallic current collector supporting an electrochemically active material on at least one of its faces. Each electrode is electrically connected to a power output terminal which provides electrical continuity between the electrode and an external application with which the cell is associated. The electrode plate group is placed in a container that is sealingly closed by a cover.
The invention relates to sealed secondary cells, of for example the primary lithium or lithium ion types. These cells may be of cylindrical format or of rectangular format (also known as prismatic format).
A cell is generally designed to operate under so-called “nominal” conditions, i.e. within given ranges of temperature, current, and voltage. When a sealed cell is used outside the nominal conditions, for example in the event of accidental overcharging, or a short circuit, or a temperature higher than the maximum operating temperature, etc., there is a danger of explosion. This is because such situations lead to electrochemical reactions with generation of gas. An accumulation of such gases inside the container increases the internal pressure of the cell which can lead to the container bursting violently and to chemical compounds that are harmful and corrosive for the environment and for people situated nearby being sprayed out.
Safety devices exist that prevent gas from accumulating inside the container of a sealed cell and that enable the gas to be exhausted when the internal pressure exceeds a predetermined value.
Further, is essential for a safety device for a secondary cell to also incorporate a circuit breaking function able to electrically isolate, irreversibly, the secondary cell from apparatus connected thereto.
French patent 2,873,495 (US 2006 0019150) discloses a sealed secondary cell. The cell consists of alternating positive and negative electrodes connected to their respective power output terminals. The electrodes are arranged in a container having a cylindrical wall closed off by a base. The base is made up of three portions, a peripheral portion connected to the cylindrical wall, a releasable closure member portion and a thinned portion connecting the closure member portion to the peripheral portion. The thinning is adapted to be torn as a result of excess pressure inside the container. A planar connection connects the positive electrode of the electrode plate group to the wall of the base of the container, the walls of the container being electrically conducting with the cover. An annular seal is located between the electrode plate group and the bottom of the container. This acts as an electrical insulator to prevent electrical contact between an edge of a negative electrode of the plate group and the wall of the container connected to the positive terminal, which could bring about a short circuit after the safety device has been actuated. Tearing of the thinning is adapted to interrupt electrical conduction between the electrodes of one polarity and their power output terminal by disconnecting the releasable closure member portion from the peripheral portion. The electrode plate group is in abutment with the connection part. In the presence of excess pressure, the thinned portion tears and the electrode plate group which bears on the releasable closure member portion via the connection part shifts towards the bottom of the container, tearing the totality of the thinned portion in order to break the electrical connection between the positive electrode of the electrode plate group and the cover of the container. FIGS. 1A and 1B respectively show a longitudinal cross-section of the lower portion of a similar container and a cross-sectional view when the closure member portion is disconnected.
Such a container nevertheless has the disadvantage of being vulnerable to jolting and impact. In effect, the electrode plate group, which is the heaviest part of the element, is lying on the releasable closure member portion. In the presence of jolts or impact, the weight of the electrode plate group exercises a force which may cause the thinning to tear thereby undesirably disconnecting the closure member portion from the peripheral portion. It has been observed that the closure member portion initially designed to open at a cell internal pressure greater than or equal to 8 bar to 12 bar (depending on cell capacity) does in reality, and not in the presence of jolts or impact, open at a lower internal pressure (between 0 and 6 bar). This can be explained by a weakening of the thinning brought about by jolting or impacts. Given that a secondary cell can have an internal pressure of 6 bar at the end of its lifetime, untoward opening of the closure member portion can occur.